The present invention generally relates to RF power amplifiers and, in particular, relates to efficient, linear RF signal amplification.
RF signal amplification represents a common step in the generation and transmission of RF signals. Typically, wireless communication devices, such as the mobile terminals used in a wireless communications network, include one or more power amplifiers that provide RF signal amplification. These power amplifiers boost the power level of the RF signal transmitted by the mobile terminals to a level appropriate for transmission to remote base stations and the like.
Mobile terminals usually have significant constraints on overall operating power. As the size of mobile terminals decreases in response to consumer demands for convenience and portability, so too does the size of included battery packs. Because the RF power amplification sections of mobile terminals represent a chief area of power consumption, efficient signal amplification is paramount to acceptable mobile terminal “talk-time” ratings. Such ratings express the length of time a given mobile terminal will operate in an active communications mode, before depleting its battery charge. Power amplifier efficiency is critical then to achieving competitive mobile terminal operating times.
The ratio of output RF power to input DC power establishes the power amplifier's efficiency rating. In general, the efficiency of a given power amplifier increases as the amplifier's output power moves upward through the linear amplification range, and efficiency reaches its maximum at a defined point in the amplifier's saturated mode of operation. While the power amplifier in a battery-powered device, such as a mobile terminal, would ideally operate in a saturated mode whenever it was actively transmitting, communication standards typically determine whether or not such operation is permissible. For example, the long-established Advanced-Mobile-Phone System (AMPS) standard defines a constant-envelope, frequency modulated transmission signal.
Absent the need to convey amplitude information, power amplifiers can operate in higher-efficiency saturated mode operation when transmitting within an AMPS system. Newer digital modulation standards, used in, for example, Enhanced Data through GSM Evolution (EDGE), employ techniques requiring some form of linear signal modulation. EDGE, for example, uses 8-ary Phase Shift Keying (PSK) modulation. These digital modulation standards are more spectrally efficient, e.g., they permit the transmission of more information within a given portion of radio frequency bandwidth. However, amplifier power efficiency suffers because of the need for linear amplification.